1. Field of the Invention
The present invention relates to a radio receiver that receives a radio signal and performs an automatic gain control on the radio signal.
2. Description of the Related Art
A general radio receiver includes an automatic gain control (AGC) function for adjusting a gain corresponding to an electric power (or a voltage) of an input signal. The AGC function increases a gain of an amplifier when an input power is smaller than a predetermined reference power (or decreases an attenuation by a variable attenuator), or decreases the gain of the amplifier when the input power is larger than the reference power, or increases the attenuation by the variable attenuator. Accordingly, the level of the received signal is kept constant to be input to an analog-digital converter (ADC), thereby realizing a wide dynamic range.
The reference power in such an AGC circuit is fixed at a constant value. Furthermore, it is necessary to set the AGC circuit to a slow time constant to some extent, so that an amplitude component of the signal does not change more than necessary. In a calculation of the reception power, the reception power is averaged over a specific long-time interval (for example, over several tens of frame lengths), and the average reception power is used for the AGC. Such an AGC technique is disclosed in, for example, “W-CDMA Mobile Communications System” by Keiji TACHIKAWA, P. 29, FIGS. 2-4, Maruzen Co., Ltd. Publishing Division.
However, the conventional AGC circuit assumes that a power difference does not occur between time slots within a monitored cycle (for example, within one frame). Therefore, when a power difference occurs between time slots within the monitored cycle, appropriate AGC control cannot be performed. Hence, the input level with respect to analog devices such as an amplifier and a mixer, and the ADC in the subsequent stage is not appropriate, thereby causing a problem in that an S/N ratio of the input signal deteriorates, or on the contrary, signal distortion or clipping occurs.
Conventionally, therefore, it has been necessary to use a high-performance analog device that can sufficiently ensure back-off so that the received signal is not distorted, and a high-performance ADC having high effective bit accuracy so as to reduce S/N deterioration due to a quantization error, to constitute a radio receiver. Accordingly, the unit price of the necessary device increases, thereby increasing the whole price of the radio receiver.